In recent years, much effort has been made to develop techniques for various electric-power conversion devices. For example, if an electric-power conversion device includes an inverter circuit, the inverter circuit is formed of a plurality of semiconductor switches. Semiconductor switches are based on electric-power switching elements. A main element in a semiconductor switch comprises a switching element for electric-power and an antiparallel diode connected in anti-parallel with the switching element. Much effort has also been made to develop techniques to improve the reverse recovery property of antiparallel diodes.
In addition to the main element, the semiconductor switch comprises a reverse voltage application circuit. The reverse voltage application circuit applies a reverse voltage lower than a withstand voltage of the main element to the antiparallel diode. The reverse voltage application circuit forms one arm of a bridge circuit.
The reverse voltage application circuit comprises an auxiliary electric-power supply providing a voltage smaller than that of the withstand voltage of the main element, an auxiliary element which is turned on at the time of reverse recovery of the antiparallel diode and which has a lower withstand voltage than the main element, a high-speed free wheeling diode having a reverse recovery time of which is shorter than the antiparallel diode, and a reverse recovery charge of which is smaller than the antiparallel diode, and a capacitor connected in parallel with the auxiliary electric-power supply. The auxiliary electric-power supply, the auxiliary element and the high-speed free wheeling diode are connected together in series.
The auxiliary element is turned on during a period of dead time, whereby a main current is commutated from the antiparallel diode to the high-speed free wheeling diode by a supply of energy from the capacitor charged by the auxiliary electric-power supply. In the state where the high-speed free wheeling diode performs return current operation, an on signal is input to a control terminal of the main element on the opposite arm. Thus, instead of the antiparallel diode, the high-speed free wheeling diode performs a reverse recovery operation. This technique, compared to conventional methods, enables a significant reduction in a surge current resulting from the reverse recovery.